diff --git a/build/pre-compile.js b/build/pre-compile.js
index 29f4ea5c7..a5cac6dda 100644
--- a/build/pre-compile.js
+++ b/build/pre-compile.js
@@ -340,7 +340,7 @@
       ];
 
       // minify `iteratorObject.keys`
-      source.replace(/\b(iteratorObject(?:\.|\['))keys\b/g, function(match, prelude) {
+      source = source.replace(/\b(iteratorObject(?:\.|\['))keys\b/g, function(match, prelude) {
         return prelude + minNames[iteratorOptions.length + props.length];
       });